Removed hardcoded paths to private key

Without this patch, the hardcoded path of the private key prevents to perform non-root executions of Oz, getting the following error:
```
DEBUG:oz.Guest.RHEL6Guest:Testing ssh connection
Enter passphrase for key '/etc/oz/id_rsa-icicle-gen': 
```
The proposed patch gets this information from oz.cfg adding a new parameter 'sshprivkey' that defaults to '/etc/oz/id_rsa-icicle-gen'